In the development of high-capacity lithium secondary batteries, since amorphous tin-based composite oxide having a theoretical gravimetric capacity more than 2 times that of carbon (372 mAh/g) was suggested as an anode active material in the year 1997, many studies have been focused on lithium-reactive metals and metal oxides, for example, Sn, Si, SnO2 and Sn2P2O7, due to their high capacities and relatively low average working potentials (<1.51V). However, such materials have several serious problems in their practical application to lithium secondary batteries.
One of such problems is that these materials have a very low reversible capacity. Specifically, a capacity loss of more than 40% occurs after the first charge/discharge cycle because of the formation of an inactive matrix phase in the lithium phase (e.g., Li2O) and strong side reactions with an electrolyte after the first cycle.
Another problem in practical application is that these materials undergo a very high volume change due to the formation of LixM and M phases during charge/discharge cycles. As a result, active material particles are pulverized, so that they are separated from a current collector, thus causing electrical isolation.
Recently, metal phosphides (MPs) have been suggested as one of the most promising electrode materials. These materials have average working potentials higher than that of Sn, Si or SnO2, but are characterized in that they can react reversibly with Li at an average voltage of 1V.
According to the nature of the metal, reactions involving metal phosphides MPn and lithium can be classified into two groups:
(1) Li intercalation                MPnLixMPn         
(2) metallization or metal alloying                MPnM(LixM)+LixP        
MnP4 is included in the first group, in which lithium intercalation occurs due to electrochemical redox processes (MnP4 Li7MnP4). Also, the P—P bonds in MnP4 are cleaved during lithium intercalation to form Li7MnP4. The formed Li7MnP4 is reoxidized to MnP4 only between 0.57 V and 1.7 V, and on the other hand, Li7MnP4 is decomposed into Mn and Li3P below 0.5V.
Even though electrodes comprising MnP4 were cycled in a limited voltage window, they showed a rapid capacity fading from 700 mAh/g to 350 mAh/g during initial several cycles and were stabilized after 50 cycles. A recent study conducted by Gillot et al. suggested that the reason for poor capacity retention in MnP4 is because Li7MnP4 is irreversibly decomposed to Li3P and Mn, instead of recrystallization of Li7MnP4 to MnP4.
Similarly, MPn compounds, such as CoP3, Cu3P, VP4 or Sn3P4 also showed decomposition reactions similar to the above reaction. Such materials also have problems of 1) particle pulverization caused by large volume changes, and 2) fast capacity fading because of the formation of low-conductive LiP from Li3P at 0.65 V or more.
Meanwhile, tin phosphides, such as SnP, SnP3 or Sn4P3 were also reported in prior literature. Such materials can be prepared by thermally treating stiochiometric amounts of tin and red phosphorus at high temperatures according to a conventional solid-phase reaction method. Recently, a technique of preparing nanometer-size Sn4P3 from tin and red phosphorus using a mechanical milling method has also be suggested.